The present technique relates generally to the field of lighting systems and, more particularly, to high intensity discharge lamps.
High intensity discharge lamps are often formed from a ceramic tubular body or arc tube that is sealed to one or more end caps or end structures. High intensity discharge lamps generally operate at high temperatures and high pressures. Because of operational limitations, various parts of these lamps are made of different types of materials. The process of joining different materials in high-temperature lamps creates significant challenges. Specifically, the different thermal coefficients of expansion of these joined materials can lead to thermal stresses and cracks during operation of the lamp. For example, thermal stresses and cracks can develop at the seal interface between the different components, e.g., arc tube, electrodes, end caps, and so forth. Certain end-cap materials used to provide favorable and reliable stress distribution in the ceramic at the end of the ceramic lamp unfortunately are not chemically resistant to halide species that may be used in the lamps, especially at elevated temperatures.
Typically, high intensity discharge lamps are assembled and dosed in a dry box, which facilitates control of the atmosphere. For example, in the controlled environment within the dry box, the lamp end-caps are attached to an arc tube with the assistance of a furnace, which is also disposed within the dry box. The assembly of seal material, end-caps and arc tube is inserted into a furnace and the furnace is operated through a controlled temperature cycle. The controlled temperature cycle is designed in conjunction with a temperature gradient at the end of the furnace to melt the seal material (typically a dysprosia-alumina-silica mixture), which then flows through the gap between components to seal the end-caps to the arc tube. Typically a furnace such as a large muffle type furnace with temperatures reaching to about 1500 degrees centigrade or higher is used. The assembly is typically held at the temperature for about 30 seconds to about 45 seconds, then the temperature of the assembly is brought down to room temperature to seal the end structures to the arc envelope. Unfortunately, this requirement of a dry box environment with a furnace disposed within the box severely limits production efficiency of the lamps. For some lamp applications, it is desirable to have a room temperature pressure of 10 to 20 atmospheres to better enable rapid start-up. Dry box processing makes it difficult to seal lamps with such high pressure fills.
Accordingly, a technique is needed to address one or more of the foregoing problems in lighting systems, such as high-intensity discharge lamps.